This invention relates to a conical scan tracking system for spacecraft and other radio sources employing a large antenna.
To actively track spacecraft and distant radio sources, it is necessary to utilize a large antenna, such as the 64-meter paraboloid antenna at Goldstone, California. A conical scan tracking system probably has the most accuracy because it continuously provides correction signals without requiring degradation of system noise temperature as caused by a monopulse feed system. A single antenna is mechanically rotated in a conical pattern at a rate f.sub.s about a pointing axis of the tracking system. If the target remains on the axis, the target signal is constant. If the target is displaced, the target signal is modulated sinusoidally with one cycle for each scan cycle. The amplitude of modulation indicates the extent of target displacement, and the phase of the modulation indicates the direction of displacement.
For small excursions from the axis, the modulation will be linear and expressed by the relationship EQU A(t) = A.sub.o [1=k.sub.s .epsilon.cos(.omega..sub.s t-.phi.)](1)
where A(t) is the amplitude received at time t, A.sub.o is the amplitude averaged over the scan cycle, k.sub.s is the "error slope" of the system, .epsilon. is the magnitude of the error, .phi. is the phase angle of the error, and .omega..sub.s =2.pi.f.sub.s. If t is taken as zero when the scan is at the point to the right of the axis, proceeding counterclockwise, then it follows that EQU A(t) = A.sub.o (1+k.sub.s .epsilon..sub.x cos.omega..sub.s t+k.sub.s .epsilon..sub.y sin.omega..sub.s t) (2)
where .epsilon..sub.x is the component of error to the right and .epsilon..sub.y is the upward component. The two error components produce independent modulation components in quadrature with each other which may be separated.
The two error components may be separated after error signal detection by use of a suitable phase-sensitive error demodulator. The .epsilon..sub.x component becomes the input to an azimuth drive servo, while the .epsilon..sub.y becomes the input to an elevation drive servo. In order to hold the error slope k.sub.s constant at the servo input for various target sizes and ranges, an automatic gain control (AGC) loop is used in the receiver. The time constant of the AGC loop is generally much smaller than the scan period, so that the modulation at the scan frequency is not suppressed. Alternatively, a slow AGC may be used and the error modulation is extracted from the detected signal voltage itself.
An accuracy of 0.001.degree. has been achieved in aircraft tracking systems using radar with a small antenna scanning at a relatively high rate of many cycles per second so that analog correlation suffices. An object of this invention is to achieve similar accuracies in a conical scan tracking system employing a large antenna, such as the 64-meter Goldstone antenna. However, because of the size of the antenna, it can be scanned only at very low rates. For a scan cycle period of ten to one hundred seconds, the "double frequency" terms arising in analog correlation would cause unwanted oscillation in the tracking system.